Regulation of biosynthesis and cellular localization of Sp32 annexins in tobacco BY2 cells

Annexins interact in a calcium-dependent manner with membrane phospholipids . Although their exact function is not known, annexins have been proposed t o be involved in a variety of cellular processes. To determine whether plan t annexins are implicated in cell division, we have isolated cDNAs encoding annexin from TBY2, cells. Based on sequence analysis, these cDNAs fall int o two families, differing mainly by deletions or insertions in their 5'- an d 3'-untranslated regions. The two annexins Ntp32.1 and Ntp32.2 encoded by these cDNAs are homologous to p.32 from bell pepper (Cap32.1): we propose t hat these Solanaceae annexins constitute a distinct type which we call Sp32 annexins. There are two genes (Ntan.1 and Ntan.2) derived from the separat e progenitor species of Nicotiana tabacum and analysis of Southern blots is consistent with the presence of these two genes. We show that Sp32 transcr ipt amounts are developmentally modulated in tobacco plants: RNA levels are highest in growing and dividing tissues. Sp32 annexin gene expression is a lso regulated in TBY2 cultured cells: transcripts and proteins are detected only in exponentially growing cells. In synchronized TBY2 cells, Sp32 anne xin transcripts are expressed at the G2/M transition, in the hi phase and a t the G1/S transition. These results are the first evidence that the expres sion of plant annexins is modulated during the cell cycle. The Sp32 annexin proteins accumulate during the cell cycle and peak at the end of mitosis. Immunolocalization shows that the majority of Sp32 annexins is present in i ntercellular junctions, forming a ring structure under the plasma membrane. Since annexins are known to bind secretory vesicles during exocytosis, the ir localization at cell junctions suggests that annexins could be involved in cell wall maturation.